Mitral stenosis is a type of valvular heart disease. The basic function of the mitral valve is to allow blood to enter the left ventricle from the left atrium when open and to ensure that blood enters the aorta from the left ventricle when closed and does not return to the left atrium, which is equivalent to a one-way valve. When mitral stenosis occurs, the valve opening is reduced and does not open properly. This restricts the circulation of blood throughout the body while causing a number of changes. It is possible for this lesion to present at birth with a smaller than normal mitral valve opening, which may be termed a congenital anomaly. Surgical treatment may be required in infancy.